1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an active matrix type liquid crystal display device capable of reducing power consumption and preventing degradation of display quality.
2. Description of the Background Art
The liquid crystal display device includes a display panel and a control circuit to control the liquid crystal display panel. The liquid crystal display panel also includes a pair of substrates facing each other, a liquid crystal layer held by the pair of substrates and a display area having a plurality of pixels arranged in a matrix. The display area includes scan lines along pixels arranged in a row direction and signal lines along pixels arranged in a column direction. Each pixel includes a pixel electrode and a switch element that connects the pixel electrode to the signal line.
Generally, the liquid crystal display device adopts an alter electric field driving, that is, a polarity of voltage applied to the liquid crystal layer is reversed in every selected scan line as countermeasure against flicker. However, if only one of the polarity changes in every selected scan line and every selected signal line is adopted, flicker may be generated in a direction in which the scan lines or the signal lines extend. Accordingly, in a high quality liquid crystal display device, a dot inversion driving in which the polarity of the voltage applied to the liquid crystal layer is reversed both in every selected scan line and every selected signal line. For example, Japanese laid open patent application No. 2003-280036 discloses such technique.
On the other hand, a capacitance coupling driving (CC driving) is proposed to decrease the amplitude of signal voltages. In the capacitance coupling driving, a predetermined pixel voltage is obtained by adding an auxiliary capacitance signal to the pixel electrode through an auxiliary capacitance. If the capacitance values of the auxiliary capacitance and a pixel capacitance are set substantially equal, the amplitude of the signal voltage may be reduced by half. Japanese laid open patent application No. 2005-49849 describes a capacitance coupling dot inversion driving (CCDI driving) in which the dot inversion driving is combined with the capacitance coupling driving (CC driving). In the liquid crystal display device, a black insertion driving in which a signal corresponding to black display is applied to the pixel to achieve a clearer picture.
When small amplitude of the signal voltage is required, the CCDI driving is adopted. Further, when the black insertion driving is conducted, two time scan, that is, a scan for the black insertion driving and a scan for writing image signals into the pixel are necessary within one frame scan period (1 vertical scan period). After finishing the scan period, the voltage of the auxiliary capacitance signal changes to the same polarity side as the written image signal. If the auxiliary capacitance signal changes to an opposite polarity side to the written signal, which results in difficulty of the alternate driving in every vertical scan period. Furthermore, when the capacitance coupling driving is conducted, the pixel electrode voltage may exceed “off” voltage of the switching element owing to superposing of the auxiliary capacitance signal voltage. In this case, a displayed image quality may be reduced due to current leak from the switching element, which results in decrease of the voltage applied to the liquid crystal layer.
On the other hand, when the black insertion is conducted, it is possible to set the superposed voltage of the auxiliary capacitance signals for the black insertion and the image display at different values each other. However, center voltages applied to the liquid crystal layer may shift to different ones when the black signal and the image signal are written into the pixel electrode. In such a case, a direct current component (DC) may be supplied to the liquid crystal layer. The shift of the center voltages may result in image persistence or display unevenness.